Bipolar plates form the structural and functional backbone of proton-exchange membrane fuel cells (PEMFCs), ensuring uniform gas distribution, efficient current collection, and removal of reaction products. Accounting for roughly 80% of a PEMFC’s mass and 20–30% of its cost, these components must withstand the demanding environment inside the cell, which includes acidic conditions (pH 3), fluoride ion exposure, and high humidity. Materials selection for bipolar plates has traditionally centered on graphite, metals, and composites. Graphite offers excellent corrosion resistance and conductivity but suffers from brittleness, porosity, and high processing costs, limiting its use in mobile applications. Composite plates, typically resin matrices reinforced with conductive fillers such as carbon fibers or nanotubes, face challenges in balancing electrical performance with mechanical strength.
Metallic bipolar plates, particularly stainless steel and titanium alloys, have gained prominence due to their superior electrical and thermal properties, mechanical robustness, manufacturability, and cost-effectiveness. Stainless steel, especially SS316L, is widely regarded as a leading candidate for automotive fuel cells. Titanium alloys, such as TA2, offer a high strength-to-weight ratio and exceptional corrosion resistance, enabling significant reductions in stack volume and weight while increasing power density. In one notable example, titanium-based plates allowed a stack volume reduction from 64 to 37 liters, weight reduction from 108 to 56 kilograms, and power density improvement to 3.1 kW/L.
Despite these advantages, metallic plates are vulnerable to corrosion in the hot, humid, and acidic environment of PEMFCs. While a passivation film naturally forms on metal surfaces, providing some protection, this layer typically exhibits poor electrical conductivity, increasing interfacial contact resistance between the plate and the gas diffusion layer (GDL) and thereby impairing fuel cell performance. Surface modification through protective coatings has emerged as a viable strategy to enhance corrosion resistance while preserving conductivity. Coatings fall into two main categories: metal-based (including noble metals and metallic carbides/nitrides) and carbon-based (such as graphite, amorphous carbon, and certain polymers). Amorphous carbon (α-C) is particularly attractive due to its chemical inertness and good electrical conductivity.
Previous work by Yi et al. employed magnetron sputtering to deposit α-C films on stainless steel plates, varying parameters such as bias voltage, deposition time, bias frequency, and argon flow to control the sp2 hybridization ratio. Their findings indicated that higher sp2 content and coating compactness improved both corrosion resistance and conductivity. Research on α-C coatings for titanium substrates has been less common. Shown et al. used plasma-assisted chemical vapor deposition with ethylene as the carbon source to coat pure titanium, observing that higher deposition temperatures enhanced film conductivity.
Amorphous carbon films produced by physical vapor deposition (PVD) can exhibit defects like pinholes, vacancies, and cracks, which allow corrosive ions to penetrate. Consequently, the substrate’s inherent properties influence the coating’s overall performance in PEMFC conditions. In the present study, α-C films were deposited on SS316L and TA2 substrates using DC balanced magnetron sputtering. Substrates were cleaned with a metal cleaning agent, ultrasonically treated with ethanol and deionized water, and oven-dried prior to coating. The sputtering process employed graphite targets at a furnace temperature of 300 °C, with substrates mounted on a rotating holder.
Surface morphology analysis revealed relatively smooth films without large defects, composed of cauliflower-like clusters 50–100 nm in diameter, with distinct boundaries between clusters. The α-C coating on SS316L exhibited different morphological features compared to TA2, reflecting substrate influence on film growth.
Electrochemical testing showed substantial improvements in corrosion resistance. The corrosion current density for SS316L dropped from 63.9 μA/cm² to 0.148 μA/cm² after coating, while TA2 decreased from 0.677 μA/cm² to 0.051 μA/cm². At 0.6 V, both coated substrates demonstrated excellent resistance. However, at 1.4 V versus the saturated calomel electrode, SS316L with α-C experienced severe localized corrosion, whereas TA2 maintained integrity without local attack. This difference underscores the role of substrate properties in determining coating durability under high potential conditions.
The passivation film formed on SS316L in PEMFC operation provided baseline protection but introduced higher contact resistance. The α-C coating mitigated this issue by maintaining conductivity while enhancing corrosion resistance, particularly on titanium substrates where the combination proved more robust under aggressive electrochemical conditions.